The present invention relates to a new type of corona discharging apparatus. In a more detailed description, the apparatus is equipped with a pair of thin plate electrodes. It is characteristic of this apparatus that the equipped electrodes are allowed to reciprocate along a definite path within a desired distance. By using the apparatus, it is able to perform corona discharge on polymer film, either in a still or moving state, for a considerable length of time without causing dielectric breakdown. One of the practical applications of the apparatus is poling of stretched PVDF (Polyvinylidene fluoride) films to gain high piezoelectric activity with high production yield.
Generally speaking, the process for manufacturing piezo-electric PVDF films is based on the poling of stretched PVDF films. Of the currently known poling techniques, the most widely employed methods are thermal contact and corona discharge, as disclosed in Chinese patent No. 32963 and in U.S. Pat. Nos. 4,365,283 and 4,392,178. Thermal contact method consists of the application of a DC electrical field to a film inserted between the surface electrodes while the assembly is heated and finally cooled to ambient temperature; the corona discharge method charges the polymer film by application of a DC voltage of a few KV between a needle or knife electrode and a back electrode located behind the film. Between the two methods, corona poling is comparatively much easier to do because it requires shorter times and can be performed at room temperature.
In the conventional corona discharging apparatus, one of the electrodes is usually of roller type. The apparatus may carry out poling continuously, and poling time is dependent on rolling speed of the roller electrode. However, there is a disadvantage that the rolling speed or film feeding speed is not easily controllable, too fast a speed may reduce the film piezoelectric activity owing to insufficient poling, while too slow a speed may cause the film to undergo dielectric breakdown due to high field strength. Thus, piezoelectric film having both high quality and high production yield is not achieveable by the conventional poling techniques.